There are two ways to look at this question—from a practical point of view and a biblical point of view. First, from a practical point of view, let’s assume that a person does contribute something to his salvation. If that were possible, who would get the credit in heaven? If we somehow contribute to our own salvation, it would follow that we get the credit. And if we get the credit, this certainly will detract from God’s getting the credit. If it were possible to contribute something to attain heaven, then people upon their arrival would be patting themselves on the back because of what they did in order to obtain heavenly citizenship. These same people would be singing, "Praise myself, I contributed to my own salvation." It is unthinkable that people in heaven will be worshiping self rather than God. God said, “I will not give my glory to another” (Isaiah 42:8; 42:11).
From a biblical point of view, mankind contributes nothing at all to his salvation. The problem with humanity is their sinfulness. Theologians normally refer to this as "total depravity." Total depravity is the belief that mankind is sinful throughout and can do nothing of himself to earn God’s favor. Because of this sinful state, mankind wants nothing to do with God (see especially Romans 1:18-32). It is safe to say that because mankind is totally depraved, mankind chooses to sin, loves to sin, defends sin, and glories in sin.
Because of man’s sinful predicament, he is in need of God’s direct intervention. This intervention has been provided by Jesus Christ, the mediator between sinful humanity and righteous God (1 Timothy 2:5). As already stated, humanity wants nothing to do with God, but God wants everything to do with us. This is why He sent his son Jesus Christ to die for the sins of humanity—God’s perfect substitution (1 Timothy 2:6). Because Jesus died, through faith mankind can be declared justified, declared righteous (Romans 5:1). By faith, the person is redeemed, bought out of the slave market of sin, and set free from it (1 Peter 1:18-19).
These acts just mentioned—substitution, justification, redemption—are just a few that are provided for completely by God, and devoid completely of anything human. The Bible is clear that mankind cannot contribute anything to his salvation. Any time someone thinks he can contribute, he is in essence working for his salvation, which is clearly against the Bible’s statements (see Ephesians 2:8-9). Even faith itself is a gift from God. Salvation is a free gift from God (Romans 6:23), and since it is a gift, there is nothing you can do to earn it. All you have to do is take the gift. "But to all who have received him (i.e., Jesus)—those who believe in his name—he has given the right to become God’s children" (John 1:12).